Hydrothermal treatment of kaolin clays to produce modified products of various types, has been known for many years, and is widely described in the prior art technical and patent literature
The reaction of sodium silicate with kaolin clays, for example, has been studied under various hydrothermal conditions, as reported by Kurbus et al, Z. Anogo. Allg. Chem., 1977, Volume 429, pages 156-161. In Kurbus et al these reactions were thus studied under hydrothermal conditions using essentially equivalent molar ratios of the kaolin and sodium silicate with the reaction being carried out in an autoclave. The products of the reactions, as identified by x-ray, electron microscope, and infrared methods, showed that sodium silicate reacts with kaolin to form an alumino-silica gel or a crystallized zeolite mineral analcime of the formula: EQU Na.sub.2 O:Al.sub.2 O.sub.3 :4SiO.sub.2 :2H.sub.2 O
In the reaction, the kaolin dissolves and alpha-quartz simultaneously appears in the product of reaction.
U.S. Pat. No. 4,812,299 issued Mar. 14, 1989 to S. K. Wason (assigned to J. M. Huber Corp.) describes compositions designated as synthetic alkali metal alumino-silicates, or simply SAMS, prepared by the hydrothermal reaction of an alkali metal silicate and kaolin clay. By the term "hydrothermal" it is meant that the reaction is carried out under aqueous conditions of elevated temperatures and pressures of greater than atmospheric. In a preferred practice of the said process, an aqueous slurry of the starting clay material and the alkali metal silicate is formed, the system is closed, and heat applied to gradually raise the temperature. The reaction is conducted under conditions of agitation. In general, the pressure in the system will range from about 50 to 360 psig at temperatures ranging from about 140 to 250 degrees C. A specifically preferred range of conditions is to operate the process at pressures of 100 to 200 psig and temperatures of 164 to 194 degrees C. The temperatures are preferably correlated to the pressure such as that provided by steam. The reaction time is about 0.25 to 4 hours. After completion of the reaction, heat is removed and the mixture is allowed to cool, after which the system is opened, the product separated by filtration or centrifugation, washed with water, and dried. Spray drying is preferred at inlet temperatures of 1000.degree. F. (538.degree. C.) and outlet temperature of 250.degree. F. (121.degree. C.). The products are defined as structured agglomerates wherein the primary particles comprise altered kaolin clay platelets integrated with one or more adjacent areas of essentially amorphous alkali metal silicate base-kaolin clay reaction products. More specifically, they are described as altered kaolin platelets having an integrated rimmed area of amorphous, non-diffracting alkali metal silicate-kaolin reaction product. The products are said to be useful as reinforcing agents or fillers for papers, paints, plastics and rubber and to have increased opacity and brightness as compared with the starting clay material.
U.S. Pat. No. 4,816,074 issued Mar. 28, 1989 to Rasik H. Raythatha et al, assigned to E.C.C. America Inc., describes a non-hydrothermal process, in which a structured aggregated kaolin pigment is prepared by mixing substantially dry kaolin in particulate form with an aqueous alkali metal silicate to deposit on the surface of the kaolin particles a substantially molecular level of said silicate without formation of silica gel, drying the treated kaolin without calcination and exposing it to an acidic gas. Calcium carbonate may be added to the composition to be treated, as an aggregation enhancing agent. The product is useful as a pigment in the coating or filling of paper. The process serves to aggregate the very fine particles (i.e. the sub 0.25 micrometer particles) present in very fine feeds. Thus, the otherwise large percentages of troublesome extremely fine particles are effectively removed as separate entities, but without the need for separation steps, together with the costly equipment required for such operations.
U.S. Pat. No. 3,765,825 issued Oct. 16, 1973 to V. J. Hurst teaches reducing the viscosity of kaolin clay slurries by hydrothermal treatment. In U.S. Pat. No. 3,769,383 issued Oct. 30, 1973, he teaches hydrothermal treatment of kaolin to produce a variety of alumino-silicate products.
U.S. Pat. No. 4,499,062 issued Feb. 12, 1985 to P. Christophliemk et al (assigned to Henkel) teaches hydrothermal decomposition of industrial waste products such as filter sludge containing aluminum silicate and/or alkali aluminum silicate, by addition of aqueous NaOH solution and sand.